Key-On System Initialization

At key-on, the BCM performs a check to see if the EBCM is responding the BCM inputs. After the ignition switch is moved to the RUN position, the BCM grounds the Diagnostic Enable line (circuit 856). When the Diagnostic Enable line is grounded, the EBCM should respond by grounding the ABS Fault line (circuit 825) until the Diagnostic Enable line is allowed to return to 12 volts. If the initialization test fails, the following BCM codes may set:

1. BCM Code B421
   
   The BCM monitors the status of the Diagnostic Enable line at all times. If the voltage state on this circuit does not match the commanded state, Code B421 will set.Faults in the Retained Accessory Power (RAP) system and chime module may also cause B421 to set. If B421 is present and code B420 is not, see BODY COMPUTER MODULE -- ALLANTE .
2. BCM Code B480
   
   If the EBCM does not respond properly to the key-on initialization test (by grounding the ABS Fault line), Code B480 will set. B421 and B480 may set at the same time on some occasions.
3. BCM CODE B481 - ANTI-LOCK FAULT
   
   BCM Code B481 will set if the ABS Fault line remains grounded for approximately 2-1 /2 minutes.This code may sometimes set in addition to codes B480 and B421.